Non-photic Information Research Articles

Electrophysiology of GABAergic transmission of single intergeniculate leaflet neurons in rat

The intergeniculate leaflet (IGL) of the thalamus constitutes a small but important part of the neural network controlling circadian activity in rodents. It appears that IGL integrates photic cues from retina with non-photic information originating from different nonspecific brain systems. Subsequently, this integrated signal is passed to the master biological clock – the suprachiasmatic nuclei (SCN). The common neurotransmitter of biological clock neural structures, the gamma-amino-butyric acid (GABA) is expressed in many, if not all, IGL and SCN neurons. Whole-cell patch clamp in vitro electrophysiological experiments were performed in order to evaluate GABA’s influence on single IGL neurons in rat. Most neurons were hyperpolarized by GABA application and this effect was caused by activation of GABAA as well as GABAB receptors. The presence of GABAB receptors in rat’s IGL has been suggested for the first time.

The NPY intergeniculate leaflet projections to the suprachiasmatic nucleus transmit metabolic conditions

The intergeniculate leaflet (IGL) is classically known as the area of the Thalamic Lateral Geniculate Complex providing the suprachiasmatic nucleus (SCN) non-photic information. In the present study we investigated whether this information might be related to the metabolic state of the animal. The following groups of male Wistar rats were used for analysis of neuropeptide Y (NPY) and c-Fos in the IGL and SCN. (1) Fed ad libitum. (2) Fasted for 48h. (3) Fasted for 48h followed by refeeding for 3h. (4) Monosodium glutamate-lesioned and 48h fasted. (5) Electrolytic lesion in the IGL and 48h fasted. The results were quantified by optical densitometry. Neuronal tracers were injected in two brain areas that receive metabolic information from the periphery, the arcuate nucleus (ARC) and Nucleus of the Tractus Solitarius to investigate whether there is an anatomical relationship with the IGL. Lesion studies showed the IGL, and not the ARC, as origin of most NPY projections to the SCN. Fasting induced important changes in the NPY expression in the IGL, coinciding with similar changes of NPY/glutamate decarboxylase projections of the IGL to the SCN. These changes revealed that the IGL is involved in the transmission of metabolic information to the SCN. In fasted animals IGL lesion resulted in a significant increase of c-Fos in the SCN as compared to intact fasted animals demonstrating the inhibitory influence of the IGL to the SCN in fasting conditions. When the animal after fasting was refed, an increase of c-Fos in the SCN indicated a removal of this inhibitory input. Together these observations show that in addition to increased inhibitory IGL input during fasting, the negative metabolic condition also results in increased excitatory input to the SCN via other pathways. Consequently the present observations show that at least part of the non-photic input to the SCN, arising from the IGL contains information about metabolic conditions.

B29 Voluntary exercise improves circadian function in a mouse model of Huntington's disease

<h3>Background</h3> The R6/2 mouse model of Huntington9s disease exhibits many aspects of the human disease, including a progressively disrupted circadian rhythm. Previously we showed that restoring the circadian rhythm in R6/2 mice pharmacologically using alprazolam and modafinil prevented the cognitive decline seen in these mice. In addition we showed that the food-entrainable oscillator was intact in R6/2 mice whose circadian rhythms had broken down. <h3>Aim</h3> To investigate whether or not voluntary, scheduled wheel running could be used to prevent circadian breakdown in R6/2 mice. <h3>Methods</h3> We used four groups of R6/2 mice housed under LD (12:12). Control mice had fixed running wheels in their cage; “Free wheel” mice had unlimited access to wheels; “Restricted wheel” mice, could use their wheels for 6 h per day from lights off. The last group of mice had wheels that were “fixed” until they showed significant disintegration of their circadian rhythm. Then, for 4 weeks, their wheels were freed so they could be used for 3 h per day from lights off. <h3>Results</h3> The rest-activity rhythm of control R6/2 mice disintegrated progressively from 12 weeks of age. Complete disintegration of their rhythm was seen by 14 weeks of age. By contrast, in the free wheel group breakdown of the rest-activity rhythm was delayed until 14–15 weeks. Their rhythm did not completely disintegrate until 18.6 weeks. Onset of disintegration of the rest-activity rhythm in the restricted wheel group was delayed until 15.5–16.5 weeks and did not disintegrate until shortly before their death at 20 weeks. When mice in the fourth group (with disintegerating rhythms) were given access to the wheel, their rest-activity rhythm was either improved, or no further disintegration was seen. If the wheel was fixed again so they could not run on it, the rest-activity rhythm improvements disappeared. <h3>Conclusion</h3> Non-photic information from daily scheduled voluntary exercise prevented deterioration of circadian rhythms in R6/2 mice.

Retraction Note: Latitude affects degree of advancement in laying by birds in response to food supplementation: a meta-analysis

Food supplementation experiments have provided considerable information about the importance of resource availability in timing reproduction. Supplemented birds usually advance breeding over non-supplemented controls. Initial observations suggested that degree of advancement in studies conducted at higher latitudes was less than in those at lower latitudes. We hypothesized that birds at high latitudes are less responsive to the “supplementary” cue of food. We tested this hypothesis using a literature-based meta-analysis of 36 papers which, because several papers presented separate data sets from different years, yielded 56 “studies.” We used step-wise regression to determine whether latitude, elevation, the duration of supplementation, and the migratory status of the species predicted the degree to which mean clutch initiation dates of food supplemented birds differed from non-supplemented controls (i.e., effect size = $$\overline {\text{X}} _{{{\text{cnt}}}} - \overline {\text{X}} _{{{\text{suppl}}}}$$ ). Consistent with our predictions, there was a significant inverse relationship between effect size and latitude: elevation, migratory status, and duration of treatment contributed little to the model. Because the response of animals’ reproductive systems to environmental information is mediated by the neuroendocrine system, we discuss two models: (1) the adaptive specialization hypothesis in which higher latitude species that experience a relatively short breeding season have evolved a reliance on photic cues while exhibiting reduced sensitivity to non-photic cues; and (2) the conditional plasticity hypothesis in which an individual might show a marked response to non-photic information if it lived at low latitudes, but be largely driven by photic cues, endogenous rhythms, or both to the relative exclusion of non-photic information if it lived at higher latitudes.

Retinal projections and neurochemical characterization of the pregeniculate nucleus of the common marmoset (Callithrix jacchus)

In mammals, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are the main components of the circadian timing system. The SCN is the site of the endogenous biological clock that generates rhythms and synchronizes them to environmental cues. The IGL is a key structure that modulates SCN activity and is responsible for the transmission of non-photic information to the SCN, thus participating in the integration between photic and non-photic stimuli. Both the SCN and IGL receive projections of retinal ganglion cells and the IGL is connected to the SCN through the geniculohypothalamic tract. Little is known about these structures in the primate brain and the pregeniculate nucleus (PGN) has been suggested to be the primate equivalent of the rodent IGL. The aim of this study was to characterize the PGN of a primate, the common marmoset (Callithrix jacchus), and to analyze its retinal afferents. Here, the marmoset PGN was found to be organized into three subsectors based on neuronal size, pattern of retinal projections, and the distribution of neuropeptide Y-, GAD-, serotonin-, enkephalin- and substance P-labeled terminals. This pattern indicates that the marmoset PGN is equivalent to the IGL. This detailed description contributes to the understanding of the circadian timing system in this primate species considering the importance of the IGL within the context of circadian regulation.

RETRACTED ARTICLE: Latitude affects degree of advancement in laying by birds in response to food supplementation: a meta-analysis

Food supplementation experiments have provided considerable information about the importance of resource availability in timing reproduction. Supplemented birds usually advance breeding over non-supplemented controls. Initial observations suggested that degree of advancement in studies conducted at higher latitudes was less than in those at lower latitudes. We hypothesized that birds at high latitudes are less responsive to the "supplementary" cue of food. We tested this hypothesis using a literature-based meta-analysis of 36 papers which, because several papers presented separate data sets from different years, yielded 56 "studies." We used step-wise regression to determine whether latitude, elevation, the duration of supplementation, and the migratory status of the species predicted the degree to which mean clutch initiation dates of food supplemented birds differed from non-supplemented controls (i.e., effect size = X(ent -- X(suppl)). Consistent with our predictions, there was a significant inverse relationship between effect size and latitude: elevation, migratory status, and duration of treatment contributed little to the model. Because the response of animals' reproductive systems to environmental information is mediated by the neuroendocrine system, we discuss two models: (1) the adaptive specialization hypothesis in which higher latitude species that experience a relatively short breeding season have evolved a reliance on photic cues while exhibiting reduced sensitivity to non-photic cues; and (2) the conditional plasticity hypothesis in which an individual might show a marked response to non-photic information if it lived at low latitudes, but be largely driven by photic cues, endogenous rhythms, or both to the relative exclusion of non-photic information if it lived at higher latitudes.

The influence of carbachol on glutamate-induced activity of the intergeniculate leaflet neurons—In vitro studies

The intergeniculate leaflet (IGL) is a very important component of the mammalian circadian timing system. One of the best known, but still barely understood functions of the IGL, is the integration of photic (retina-derived) and non-photic information, conveyed to the suprachiasmatic nucleus (SCN)- the site of the circadian pacemaker. Glutamate, the main neurotransmitter released from the axonal endings of the retinal ganglion cells to the SCN and most probably to the IGL, is thought to be responsible for mediating the effects of light on the circadian clock. The influence of carbachol, a non-specific cholinergic agonist, on locomotor activity, c- fos expression in the SCN, and the activity of this structure has been previously studied. However, no information is available concerning the influence of acetylcholine on the activity of the IGL neurons. Therefore, the purpose of the present study was to analyze the influence of carbachol (equivalent of non-photic stimulus) on the glutamate-induced activity of the IGL neurons. Experiments were performed on thalamic rat brain slices, using extracellular, single unit recordings. After reaching a stable response to focally applied glutamate, carbachol was added to the recording medium. In the presence of the cholinergic agonist, glutamate-induced activity was decreased in 32% and increased in 13% of investigated cases. Carbachol failed to evoke any change in glutamate-induced activity in 55% of the recording cells. Our results are in agreement with previous, mainly behavioral studies, where the influence of non-photic stimulus on photic-induced changes in circadian locomotor activity was determined.

Intergeniculate leaflet: Contributions to photic and non-photic responsiveness of the hamster circadian system

The circadian visual system is able to integrate light energy over time, enabling phase response and Fos induction in the suprachiasmatic nucleus to increase in proportion to the total energy of the photic stimulus. In the present studies, the contribution of the intergeniculate leaflet to light energy integration by the hamster circadian rhythm system was evaluated. Fos protein is induced in intergeniculate leaflet neurons at much lower irradiance levels than seen in suprachiasmatic nucleus neurons. Bilateral N-methyl- d-aspartate lesions of the intergeniculate leaflet decreased phase response of the circadian locomotor rhythm to high irradiance and, in animals exposed to long duration light stimuli, reduced Fos induction in the suprachiasmatic nucleus. Normal photon integration, as indicated by attenuated rhythm phase shifts and Fos induction in suprachiasmatic nucleus cells in response to the energy in light stimuli, does not occur in the absence of the intergeniculate leaflet and is likely to be a property of the circadian rhythm system, rather than solely of the suprachiasmatic nucleus. Anatomical analysis showed that virtually no intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus contain Fos induced by either light or locomotion in a novel wheel. However, cells projecting to the pretectum were found to contain novel-wheel induced Fos. The intergeniculate leaflet is implicated in the normal assessment of light by the circadian rhythm system, but the circuitry by which either photic or non-photic information gains access to the suprachiasmatic nucleus may be more complex than previously thought.

Pinealectomy Reduces Optic Nerve But Not Intergeniculate Leaflet Input to the Suprachiasmatic Nucleus at Night

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates circadian rhythms in mammals. It receives, among others, direct inputs from the retina and from the thalamic intergeniculate leaflet (IGL). The former sends photic signals to the SCN, whereas the latter probably integrates photic and nonphotic information. To characterise these inputs in vivo, extracellular single-unit recordings were made from the SCN of rats under urethane anaesthesia during electrical stimulation of the optic nerve (OptN) or the IGL region. Cell responses were evaluated by creating peri-stimulus time histograms. Because humoral signals such as melatonin might modulate the activity of the SCN in addition to neural inputs, recordings were also made using pinealectomised (Px) rats to test for a possible role of this hormone in regulating inputs to the SCN. A significantly greater number of cells responded to IGL (60 of 90, 67%) than to OptN (35 of 75, 47%) stimulation in intact animals (chi(2) = 5.905, P = 0.015). The same was true when Px animals were tested (IGL, 82 of 131, 63%; OptN, 31 of 111, 28%; chi(2) = 27.637, P < 0.001). In intact animals, the proportion of cells responsive to IGL stimulation during the day and during the night was not significantly different from the proportion responsive in Px animals. The same was true for OptN stimulation during the day. However, during the night, the proportion of cells responsive to OptN stimulation in intact animals was significantly greater than the proportion responsive in Px animals (chi(2) = 7.127, P = 0.008). Our findings suggest that a lack of melatonin modulates OptN but not IGL inputs to the SCN.

Dexras1 Integrates Photic and Nonphotic Input

Many organisms exhibit rhythmic behavior and activity that corresponds to a daily cycle. The circadian clock is influenced by environmental factors that include both light (photic) and nonphotic stimuli (such as food and locomotor activity) (see Van Gelder). Cheng et al . created mice deficient in dexras1 , which encodes a guanosine triphosphatase (GTPase) that can also act as a guanine nucleotide exchange factor (GEF) for heterotrimeric G protein α subunits of the G i/o family. Analysis of the behavior patterns of the mice, along with various ex vivo assays of neurons of the suprachiasmatic nucleus (SCN), indicated that Dexras1 plays a key role in transmitting photic information through activation of glutamate receptors of the N -methyl-D-aspartate (NMDA) type and nonphotic information through the neuropeptide y (NPY) pathway. Phosphorylation of ERK1 and ERK2 (extracellular signal-regulated protein kinases 1 and 2) in response to light entrainment was diminished in dexras1 —/— mice. Light activates glutaminergic neurons in the retina, which synapse onto NMDA-expressing neurons of the SCN. NMDA-induced phase shifts in isolated SCN neuron firing were diminished, and in neurons from wild-type animals, these phase shifts were blocked by pharmacologic inhibition of ERK signaling. Pertussis toxin, which blocks Gα signaling, did not affect NMDA-induced phase shifts in either wild-type or dexras1 —/— mice. Thus, Dexras1 participates in a G protein-independent signaling pathway, involving NMDA receptor and ERK activation, to allow input from photic signals to the circadian clock. On the nonphotic input side, Dexras1 appears to provide a negative input, such that dexras1 —/— mice exhibit altered circadian behavior patterns after nonphotic stimulation that does not cause a similar alteration in circadian patterns in wild-type mice. In slice preparations, SCN neurons from dexras1 —/— mice were more responsive to NPY signaling, with NPY inhibiting NMDA-induced phase shifts at lower doses than in wild-type mice. NPY receptors couple to G i/o proteins, and preparations from the dexras1 —/— mice were indistinguishable from wild-type in the effect of NPY on NMDA-induced phase shifts. Thus, Dexras1 appears to provide an inhibitory signal through a G i/o -dependent mechanism to the NPY pathway for nonphotic signaling to the circadian clock. R. N. Van Gelder, Resetting the clock: Dexras1 defines a path. Neuron 43 , 603-604 (2004). [Online Journal] H. Y. Cheng, K. Obrietan, S. W. Cain, B. Y. Lee, P. V. Agostino, N. A. Joza, M. E. Harrington, M. R. Ralph, J. M. Penninger, Dexras1 potentiates photic and suppresses nonphotic responses of the circadian clock. Neuron 43 , 715-728 (2004). [Online Journal]

Inhibition of hibernation by exercise is not affected by intergeniculate leaflets lesion in hamsters.

The circadian clock of mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, has been demonstrated to integrate day length change from long (LP) to short photoperiod (SP). This photoperiodic change induces in Syrian hamsters a testicular regression through melatonin action, a phenomenon that is inhibited when hamsters have free access to a wheel. The intergeniculate leaflets (IGL), which modulate the integration of photoperiod by the SCN, are a key structure in the circadian system, conveying nonphotic information such as those induced by novelty-induced wheel running activity. We tested in hamsters transferred from LP to a cold SP the effects of wheel running activity on a photoperiod-dependent behavior, hibernation. Lesions of the IGL were done to test the role of this structure in the inhibition induced by exercise of photoperiod integration by the clock. We show that wheel running activity actually inhibits hibernation not only in sham-operated animals, but also in hamsters with a bilateral IGL lesion (IGLX). In contrast, IGL-X hamsters without a wheel integrate slower to the SP but hibernate earlier compared with sham-operated animals. Moreover, some hibernation characteristics are affected by IGL lesion. Throughout the experiment at 7 degrees C, IGL-X hamsters were in hypothermia during 18% of the experiment vs. 32% for sham-operated hamsters. Taken together, these data show that the IGL play a modulatory role in the integration of photoperiodic cues and modulate hibernation, but they are not implicated in the inhibition of hibernation induced by wheel running activity.

Dorsal raphe nucleus modulates neuronal activity in rat intergeniculate leaflet

Serotonergic input from midbrain raphe nuclei is believed to have a significant effect on mammalian circadian timing system. The suprachiasmatic nucleus (SCN) receives its serotonergic input from the median raphe nucleus, while the intergeniculate leaflet (IGL) receives serotonergic innervation from the dorsal raphe nucleus (DRN). The present paper was aimed at determining whether projection from the DRN affected rhythmic neuronal oscillations in the IGL of rats. We investigated the impact of electrolytic lesions and electric stimulation of the DRN on spontaneous isoperiodic (i.e. burst firing with a constant interburst interval) neuronal activity recorded in the IGL. In all our experiments a complete lesion of the DRN always caused a significant increase (ca. 100%) of spontaneous activity of IGL neurons, their oscillatory character having been maintained, though. On the other hand, electric stimulation of the DRN produced a transient decrease in firing rate oscillations of the IGL neurons. The obtained results indicate that the neuronal projection from the DRN has a substantial modulating effect on IGL activity-an important element of the mechanism of the circadian time-keeping system that mediates the transfer of non-photic information to the SCN by modulating its activity. The observed increase of isoperiodic activity in the IGL after DRN lesion and a transient decrease in this activity after electric stimulation indicate an inhibitory character of this effect. The present findings corroborate the hypothesis that the DRN is a one of the major and extremely important source of the modulatory inputs to the mammalian circadian time-keeping system.

Afferent projections to the hamster intergeniculate leaflet demonstrated by retrograde and anterograde tracing

The intergeniculate leaflet (IGL) is considered involved in nonphotic shifting of the circadian clock through a direct connection, the geniculo-hypothalamic tract. The brain areas mediating nonphotic arousal to the hamster IGL have not been thoroughly investigated by both retrograde and anterograde tracing. We, therefore, reinvestigated the IGL afferent connections with the retrograde tracer Cholera toxin B and subsequently verified the results with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We also defined a subset of neurons projecting to the IGL that were activated by arousal using c-Fos immunocytochemistry. Apart from a dense afferent projection from the retina- and the contralateral leaflet, there were ipsilateral projections from other structures: layer V and VI of the prefrontal cortex, the zona incerta, the magnocellular part of the subparafascicular nucleus, the dorsal raphe nucleus, the locus coeruleus, and the cuneiform nucleus. Dense bilateral projections to the leaflet from the pretectal nuclei were found. Hypothalamic afferents were observed dorsal to the suprachiasmatic nuclei, in the retrochiasmatic area (RCh) and in the ventromedial hypothalamic nuclei. All of these projections were confirmed by anterograde tracing. Furthermore, arousal (wheel-running) induced c-Fos in neurons projecting to the IGL (prefrontal cortex, RCh, pretectum). Taken together, the data strengthen the view that the IGL integrates photic and nonphotic information.

Effects of intergeniculate leaflet lesions on circadian rhythms in the mouse

We investigated effects of bilateral electrolytic intergeniculate leaflet (IGL) lesions (IGLX) on circadian rhythms of the wheel-running activity in mice kept under constant darkness conditions (DD). As a result of complete bilateral IGL lesions, the period of a free-running rhythm was lengthened by ca. 45 min in comparison with a pre-operational one. The present findings confirm results obtained previously with other animals. However, for the first time locomotor activity levels were precisely assessed in mice (the number of wheel revolutions/day) before and after IGL lesions. A computer analysis of data showed a considerable decrease in that activity (by 68% on an average) in mice with complete bilateral IGL lesions in comparison with IGL-non-lesioned individuals. The obtained results show that also in mice IGL constitutes an anatomically important element of the mechanism of a circadian time-keeping system, which mediates the transfer of non-photic information to the suprachiasmatic nuclei (SCN) by modulating their activity.

The Neuropeptide Y Y5 Receptor Mediates the Blockade of “Photic-Like” NMDA-Induced Phase Shifts in the Golden Hamster

Circadian or daily rhythms generated from the mammalian suprachiasmatic nuclei (SCN) of the hypothalamus can be synchronized by light and nonphotic stimuli. Whereas glutamate mediates photic information, nonphotic information can in some cases be mediated by neuropeptide Y (NPY) or serotonin. NPY or serotonin can reduce the phase-resetting effect of light or glutamate; however, the mechanisms and level of interaction of these two kinds of stimuli are unknown. Here we investigate the effect of NPY on the NMDA-induced phase shift of the hamster SCN circadian neural activity rhythm by means of single-unit recording techniques. NMDA (10-100 microm) applied in the early subjective night induced phase delays in the time of peak firing, whereas doses in the millimolar range disrupted firing patterns. The NMDA-induced phase delay was blocked by coapplication of NPY (0.02-200 microm). NPY Y1/Y5 and Y5 receptor agonists, but not the Y2 receptor agonist, blocked the NMDA-induced phase delay in a similar manner as NPY. The coapplication of a Y5 but not Y1 receptor antagonist eliminated NPY blockade of NMDA-induced phase delays, suggesting that the Y5 receptor is capable of mediating the inhibitory effect of NPY on photic responses. These results indicate that nonphotic and photic stimuli may interact at a level at or beyond NMDA receptor response and indicate that the Y5 receptor is involved in this interaction. Alteration of Y5 receptor function may therefore be expected to alter synchronization of circadian rhythms to light.

Species Differences Between Hamsters and Rats with Regard to the Putative Role of Serotonin in the Circadian System

In mammals, circadian rhythms of locomotor activity and many other behavioral and physiological functions are controlled by an endogenous pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Among various other afferents, the SCN receives a dense serotonergic input from the mesencephalic raphe complex. Experimental evidence obtained so far in Syrian hamsters suggests that serotonin (5-HT) mimics the effect of nonphotic stimuli during subjective day and modulates photic input to the SCN during subjective night. These findings are consistent with a putative role of serotonergic pathways in the transmission of the state of arousal to the SCN. In this paper, we review recent evidence for different modes of 5-HT action and/or the involvement of different 5-HT receptor subtypes in hamsters and rats. In intact rats, 5-HT agonists induce photic-like phase shifts of locomotor activity and melatonin rhythms as well as c-Fos expression in the ventral SCN. These results suggest a role for 5-HT in the transmission of photic rather than nonphotic information to the rat SCN. Such a function of 5-HT would also explain why the circadian system of rats is less sensitive or even insensitive to nonphotic stimuli.

Ultradian rhythmic neuronal oscillation in the intergeniculate leaflet.

Our paper is the first to describe ultradian rhythmic neuronal oscillation in the intergeniculate leaflet (IGL) of the rat. We recorded a multiple-unit neuronal activity (MUA) from dorsal to ventral parts of the lateral geniculate nucleus (LGN) in anaesthetized rats. In all the subdivisions of the lateral geniculate complex we observed spontaneous irregular firing rates of cells. However only at the anatomical localisation of the IGL, after the light was on, those responses exhibited burst firing with a constant interburst interval, which lasted several hours until the light was off. The duration of that rhythmic oscillation obtained by means of Fourier's analysis was approximately 124 s. To date we have not had sufficient data to discuss possible mechanisms of this neuronal rhythmicity. We can only conclude that light is the most important stimulus not only for suprachiasmatic nuclei (SCN), but also for the IGL. On the other hand, we can neither exclude nor confirm that in order to evoke ultradian rhythmical oscillation in the IGL, in addition to light also non-photic information is necessary.

The effect of brainstem stimulation on the evoked potentials in the intergeniculate leaflet

We have investigated the effect of the laterodorsal tegmental (LDTg) stimulation on evoked potentials in the intergeniculate leaflet (IGL) of the rat, in order to characterize how non-specific systems of the brain, whose activity indicates the influence of non-photic information, impact the activity of the IGL. IGL responses were evoked by electrical stimulation of contralateral suprachiasmatic nuclei (SCN). The amplitude of the evoked potentials was, in all experiments, significantly reduced after the LDTg stimulation. This effect indicated strong neuronal integration between the brainstem reticular formation and the IGL. These results are discussed in relation to the putative role of GABAergic projection.

Distribution of Ca 2+-dependent protein kinase C isoforms in the suprachiasmatic nucleus of the diurnal murid rodent, Arvicanthis niloticus

The suprachiasmatic nuclei (SCN) contain the major `biological clock' in mammals that controls most circadian rhythms expressed by these animals. The functional importance of protein phosphorylation and intracellular Ca 2+ in the mammalian circadian pacemaker is becoming increasingly apparent. Here we report the immunocytochemical localization of the four Ca 2+-dependent protein kinase C (PKC) isoforms (α, βI, βII, γ) within the SCN of the diurnal murid rodent, Arvicanthis niloticus, and the nocturnal golden hamster. In the SCN of A. niloticus, PKCα was the most abundant of the four isoforms. Cells containing PKCα were homogeneously distributed throughout the SCN. PKCβI cells were sparsely distributed in the perimeter of the SCN and were absent in its central area. PKCβII and -γ were not found in the SCN of A. niloticus. In the SCN of the golden hamster, PKCα cells were most heavily concentrated in the dorsomedial region, though some were also present laterally and ventrally. The distribution of arginine–vasopressin (AVP) cells in the SCN overlapped with that of PKC in both species. Species differences in the location of the Ca 2+-dependent PKC isoforms suggest differences in function such as the relaying of photic or non-photic information to the clock mechanism, or the synchronization of AVP neurons and their subsequent output signals.

Distribution of neuropeptide Y receptor expression in the rat suprachiasmatic nucleus

Neurones of the suprachiasmatic nucleus constitute the mammalian circadian clock which receives photic information via the retino-hypothalamic tract and to some extent non-photic information via the geniculo-hypothalamic tract. The majority of neurones in the geniculo-hypothalamic tract contains neuropeptide Y and both in vitro and in vivo physiological experiments have demonstrated that neuropeptide Y administered directly into the suprachiasmatic nucleus has the capacity to phase-shift the endogenous circadian rhythm of these neurones. The recent cloning of multiple mammalian neuropeptide Y receptors enabled us to perform an in situ hybridization histochemical study identifying expression of distinct neuropeptide Y receptor encoding mRNAs in the suprachiasmatic nucleus. It was seen that Y1 and Y5 receptor mRNA is highly expressed in neurones of the ventrolateral portion of the suprachiasmatic nucleus while neither Y2 nor Y4 receptor mRNA could be detected in the nucleus. These experiments demonstrate that post-synaptic neuropeptide Y mediated events in the suprachiasmatic nucleus are likely to be mediated by either of these receptors.